Cleaner usage of get_proper_type()

[ilevkivskyi]

This is a follow up for #13297

I move around some calls to get_proper_type() to preserve original types as much as possible (plus few related required low-risk changes). This makes error messages much more concise, before some error messages in the tests I added were truly epic:

Incompatible types in assignment (expression has type "Sequence[Union[B, Sequence[Union[B, Sequence[Union[B, Sequence[Union[B, Sequence[Union[B, Sequence[Union[B, NestedB]]]]]]]]]]]]", variable has type "int")

cc @JukkaL

[ilevkivskyi]

Btw I think that I checked and "optimized" completely the usage of get_proper_type() in the following files:

• applytype.py
• argmap.py
• binder.py
• erasetype.py
• expandtype.py
• join.py
• meet.py
• solve.py
• subtypes.py

Plus also some important places in checker.py and checkexpr.py. There is definitely still room for improvement, but I think it can be done later (e.g. as requested by users).

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

paasta (https://github.com/yelp/paasta)
- paasta_tools/instance/kubernetes.py:643: error: List item 1 has incompatible type "Task[List[TypedDict({'name'?: str, 'type'?: str, 'replicas'?: int, 'ready_replicas'?: int, 'create_timestamp'?: int, 'git_sha'?: str, 'image_version'?: Optional[str], 'config_sha'?: str, 'pods'?: Sequence[Mapping[str, Any]]})]]"; expected "Future[Dict[str, Any]]"
+ paasta_tools/instance/kubernetes.py:643: error: List item 1 has incompatible type "Task[List[KubernetesVersionDict]]"; expected "Future[Dict[str, Any]]"

[ilevkivskyi]

TBH, I am not 100% sure how to explain the change in mypy_primer (probably my meet.py fix), but I definitely like the new error message.

[hauntsaninja]

Everything in this PR looks great, I didn't check to see if there are changes elsewhere that could be made